The invention relates to an electric lamp provided with a lamp envelope which is sealed in a vacuum-tight manner and has a wall through which pass current-supply conductors extending to a light source arranged inside the lamp envelope; and more particularly to such a lamp having a ceramic molding which is included at least inside and is fixed to part in a lamp cap the lamp envelope being fixed to the molding, and the current-supply conductors, in at least one of which a fuse wire is included, extending through the ceramic molding to contacts on the lamp cap.
Such lamps are commercially available. They are mostly constructed as halogen incandescent lamps and are used inter alia for illumination in studios and theatres.
The lamps are provided with a fuse in order to interrupt the current through the lamp when very high currents are produced. When the fuse becomes operative, a discharge arc can be obtained which can flash over to the other current conductor or, when a lamp cap with a metal sheath is used, to the sheath of the lamp cap. This may cause a fuse to open a circuit in the electrical equipment to which the lamp is connected, or the lamp cap to become fixed by welding in the lamp holder, and/or the lamp to explode.
It is known from Dutch Patent Specification No. 51431 to include in an incandescent lamp for general illumination purposes, in which the lamp cap is joined to the lamp envelope by means of cement without the interposition of a ceramic molding, a fuse in the mass of cement. For this purpose a considerable part of the lamp cap has to be filled with cement. It has been found that such a construction is unreliable because the fuse is embedded in an uncontrollable manner. The mass of cement assumes its ultimate form after the lamp envelope and the lamp cap have been joined. It has been found that gaps may be formed in the mass of cement through which a discharge arc can flash over to the sheath of the lamp cap or to the other current conductor.
It is also known (for example from British Pat. No. 830,360) to include in a lamp for general illumination purposes a fuse wire in a glass envelope sealed in a vacuum-tight manner. However, this construction requires the manufacture of an additional component, i.e. the enveloped fuse wire, and is consequently expensive.